This invention relates to gyroscopes.
The invention is more particularly concerned with dynamically tuned gyroscopes.
Dynamically tuned gyroscopes have an inertia member such as a ring which is suspended, for limited displacement about two axes, relative to a rigidly-mounted rotating shaft. The inertia ring is rotated with the shaft and at its tuned speed effectively becomes decoupled from the shaft, behaving as though freely suspended in space.
Previous dynamically tuned gyroscopes have a central shaft that is driven for rotation about its axis by a suitable motor, and that is rigidly mounted with the gyroscope casing by means of, for example, a pair of ball-race bearings. In such gyroscopes the shaft projects at one end beyond the bearings so as to overhang them, and this projecting end of the shaft supports a gimbal ring coaxial with the shaft. The gimbal ring is resiliently mounted with the shaft by some form of torsional spring (such as, for example, a pair of flexure pivots) extending between opposite sides of the shaft and the inner surface of the ring, so that it is free for angular displacement, to a limited extent, about an axis at right angles to the shaft axis. The gimbal ring, in turn, supports the inertia ring by means of torsional springs that extend between the outer surface of the gimbal ring and the inner surface of the inertia ring, and that are located orthogonally with respect to the springs supporting the gimbal ring. In this way, the inertia ring is free for angular displacement relative to the gimbal ring about an axis at right angles to the axis of rotation of the gimbal ring relative to the shaft. The inertia ring is thereby free for rotation about two orthogonal axes relative to the axis of rotation of the shaft.
In operation, the inertia ring spins at high speed about the shaft axis and, because of its angular momentum, tends to resist changes in its attitude. If the gyroscope is displaced through a small angle relative to a non-rotating reference frame, the inertia ring will tend to maintain the attitude it had prior to displacement, thereby causing relative displacement between the inertia ring and the gimbal ring, and between the gimbal ring and the shaft. There is, in effect, a dynamically induced spring rate between the inertia ring and the shaft. It can be shown that this spring rate has a negative coefficient and is a function of the gimbal ring inertia and the square of the rotational speed of the shaft. If the positive spring rate of the torsional spring is chosen to cancel the dynamically induced negative spring rate, then the behaviour of the inertia ring is approximately that of a free rotating body in space.
In practice, because the inertia and gimbal rings can only be displaced through small angles relative to one another, and to the shaft, the gyroscopes usually function in a feedback fashion whereby a measure is taken of the force required to maintain the inertia ring in the same position relative to the gyroscope casing. This is usually done by supplying current to torque coils acting on the inertia ring, and measuring the current necessary to maintain a constant, preferably near zero, output from displacement pick-offs associated with the ring.
Such previous arrangements have several disadvantages which arise particularly from the manner in which the inertia ring is mounted relative to the shaft. The main disadvantage arises from the overhung configuration of the inertia ring whereby it is mounted at one end of the shaft outside the two bearings supporting the shaft. With this configuration, the mass of the gimbal ring, the inertia ring, the shaft and the torsional springs exerts a force on the bearings that is increased by the leverage of the shaft. This can cause strain on the bearings, especially at high speed, and also makes the assembly more susceptible to vibration effects and noise. With this overhung configuration, cantilever displacement of the inertia ring mounting caused by, for example, vibration, acceleration or gravity will give a differential output from the pick-offs, of the same kind as would occur with rotation of the gyroscope about its input axes. This results in a correction current being supplied to the torque coils and therefore in an error in the output of the gyroscope. Previous configurations are also relatively bulky and of complex construction.